I do remember running into this problem a year or two ago...
There was something about Poser calculating displacement bounds based on the materials displacement value.
Your value of 21 could be the materials displacement value times 12.

Funny thing was, Poser did change values when using a prop with multiple materials, but did not change values when using a prop with preview material only (e.g. box).

Looks like this has not been adressed in one of the Service Releases.

 

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